The object of this research is to study, in different in vitro systems and in an appropriate animal tumor model, some of the phenomena pertaining to cell differentiation, tumor maturation and stroma induction, as expressed morphologically, biochemically and immunologically, in neoplasms of the central and peripheral nervous system. The tumors investigated include human neoplasms; experimental rat tumors obtained after transplacental ethylnitrosourea (ENU) exposure; an established glial rat tumor cell line (C-60); and the neuroepithelial component of a transplantable mouse testicular teratoma (OTT-6050) maintained in syngeneic animals in ascitic and solid forms. Two main approaches are used. (1)\The establishment of suitable organotypic culture systems that permit the sequential study of differentiation by morphological, immunological and immunoradiometric methods. Autoradiographic studies are being undertaken to determine the kinetics parameters applicable to these cultures, particularly in reference to the growth fraction that can be estimated in the steady-state regions of the explants. (2)\The biological properties and in vivo and in vitro behavior and manipulation of the mouse transplantable teratoma as a model for neuroepithelial neoplastic differentiation. The interest of this model lies in its applicability to the study of embryonal central nervous system tumors in humans. Markers of differentiation include those concerned with gliofibrillogenesis (GFA protein) and neurofibrillogenesis, cell surface membrane antigens specific for neuroepithelial cells, melanin production in association with primitive neuroepithelium and the determination and quantitation of biogenic amines by highsensitivity microspectrofluorometry.